A flue gas or a diesel engine exhaust gas contains particulate matter (PM) (e.g., soot) that causes air pollution. A filter (diesel particulate filter (DPF)) formed of a ceramic or the like has been widely used to remove particulate matter. A DPF formed of a ceramic can be used for a long period of time, but may suffer defects (e.g., cracks or erosion) due to thermal deterioration or the like so that a small amount of particulate matter may leak from the DPF. It is very important to immediately detect such defects (i.e., abnormality) from the viewpoint of preventing air pollution.
As a method of detecting such defects, a particulate matter detecting apparatus may be provided on the downstream side of a DPF (see Patent Document 1, for example).
Patent Document 1: JP-A-60-123761